The esterification of 2-ethylhexanoic acid with TMPD di-2-ethylhexanoate forms the diester TMPD di-2-ethylhexanoate and water (see FIG. 1). An intermediate monoester (TMPD mono-2-ethylhexanoate) is formed during the reaction, which is subsequently converted to the diester (shown in FIG. 2). In order to achieve reasonable production rates, the reaction is conducted in the temperature range of 190° C. to 210° C. At 200° C., it takes approximately 24 hours to fully complete the conversion of the reactants to the product. Due to the slow rate of the reaction, it can be said that it is controlled by the thermodynamic rates of the system and not by any mass or heat transfer limitations.
For esterifications of this type, it is customary to accelerate the reaction by adding an excess of one of the reactants. For the reaction described above, 100% excess 2-ethylhexanoic acid is added to the reactor. The reaction can also be accelerated by continuously removing water from the system, thus taking advantage of Le Chatelier's principal to drive the reaction in the forward direction. This is done by sparging nitrogen through the reactor to absorb and remove water from the vessel. A third way of increasing rates is by increasing the temperature of the reaction. It was found that the reaction was accompanied by a degradation mechanism which produced large quantities of the dehydrated monoester (shown in FIG. 3). The formation of this dehydrated monoester presents several problems. Firstly, a yield loss problem exists making it necessary to use more reactants to produce a unit of product. Secondly, a separation problem necessitates separation of the dehydrated monoester from the product before it can be sold. Thirdly, it was further found that the rate of formation of the dehydrated monoester is more sensitive to temperature than the main reaction. Thus, one of the key variables to increase the rate of reaction had to be moderated in order to minimize degradation yield loss. The present invention seeks to overcome these problems.